All-printed paper memory

ABSTRACT

All-printed paper-based substrate memory devices are described. In an embodiment, a paper-based memory device is prepared by coating one or more areas of a paper substrate with a conductor material such as a carbon paste, to form a first electrode of a memory, depositing a layer of insulator material, such as titanium dioxide, over one or more areas of the conductor material, and depositing a layer of metal over one or more areas of the insulator material to form a second electrode of the memory. In an embodiment, the device can further include diodes printed between the insulator material and the second electrode, and the first electrode and the second electrodes can be formed as a crossbar structure to provide a WORM memory. The various layers and the diodes can be printed onto the paper substrate by, for example, an ink jet printer.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of and priority to U.S. ProvisionalPatent Application No. 62/098,629 entitled “All-Printed paper Memory”filed on Dec. 31, 2014, which is expressly incorporated by reference asif fully set forth herein in their entirety.

BACKGROUND

Due to developments in modern microelectronics, a variety of electronicdevices, such as optical discs, hard disk drives, and semiconductor orflash memory devices, are being replaced with new types of memorydevices. Recently, with new cheaper, simpler, and more energy-efficientmicroelectronic fabrication processes, attention has been focused onmaking electronic devices using printing techniques on various types ofsubstrates, especially flexible substrates.

SUMMARY

According to the embodiments described herein, paper can be relied uponas a means to store data. Recently, with new cheaper, simpler, and moreenergy-efficient microelectronic fabrication processes, attention hasbeen focused on making electronic devices using printing techniques onvarious types of substrates, especially flexible substrates. Demand forpaper-based substrate electronics may be high because of itsflexibility, foldability, low cost, mass producibility, disposability,retrievability, and ease of processing, among other factors. Varioustypes of electronic components have been implemented on papersubstrates, including wires, resistors, capacitors, transistors, diodes,etc. However, the ability to fully implement and activate paper-basedsubstrate memory devices, a key component in various circuits, is stillneeded.

Certain challenges in manufacturing paper-based substrate memory devicesstem from the difficulties of device fabrication on paper substrates.Typically, memory devices require thin and uniform layers, but papersubstrates are rough, porous, and fibrous. Moreover, although volatilememories may be more easily fabricated on paper substrates, embeddedpower solutions for such volatile memories may be unrealistic inpaper-based electronic systems. In this context, the ability tomanufacture suitable non-volatile paper-based substrate memory deviceswould be desirable.

Resistive random access memory (RRAM, or sometimes ReRAM), an emergingtype of non-volatile memory, presents potential as a suitable solutionfor paper-based memory device applications. RRAM includes one or morememory cells in a memory device, e.g., a semiconductor memory chip. Amemory cell includes a bottom electrode, a switching medium and a topelectrode according an embodiment of the present disclosure. Theswitching medium, that can be comprised of an insulator material, canexhibit a resistance that can be selectively set to various values, andreset, using appropriate control circuitry. The memory cell can be atwo-terminal resistive memory device, e.g., resistive random-accessmemory (RRAM), in the present embodiment.

The resistive memory cell can be a two-terminal memory cell having theswitching medium provided between top and bottom electrodes. Theresistance of the switching medium can be controlled by applying anelectrical signal to the electrodes. The electrical signal may becurrent-based or voltage-based. As used herein, the term “RRAM” or“resistive memory cell” refers to a memory cell or memory device thatuses a switching medium whose resistance can be controlled by applyingelectrical signal without ferroelectricity, magnetization and phasechange of the switching medium. The changes in resistance may beidentified by an associated measurement circuit and coded into binary(or other multi-level) states (e.g., 0, 1, 00, 01, 10, 11, . . . etc.).Structural simplicity is an advantage of RRAM, as only one insulator andtwo electrodes are required.

The realization of paper-based substrate memory devices relies uponfabrication techniques that are compatible with paper substrates.Conventional techniques, such as chemical vapor deposition andsputtering, usually require vacuum and/or high temperature conditions.Most of these fabrication processes are not suitable for paper-basedsubstrate memory device fabrication. Alternatively, printingtechnologies, such as screen printing, inkjet printing, micro-contactprinting, and 3D printing, are more efficient and flexible fabricationprocesses. Further, these printing technologies are appropriate forpaper-based substrate electronics and potentially other devices,including batteries, wearable antennas, supercapacitors, nanogenerators,and displays, for example. In addition, paper-based devices can berealized at relatively lower cost as compared to conventionalcomponents.

According to the embodiments described herein, paper-based substratememory devices (“paper-based memory devices,” “paper-based memory,” or“paper memory”) and methods of making the devices are provided. Thepaper-based memory can be embodied as an all-printed paper memoryincluding a metal-insulator-conductor structure. The metal layer canserve as a top electrode. The insulator layer can serve as the switchingmedium. And the conductor layer can serve as a bottom electrode. Themetal-insulator-conductor structure can be formed from structure ofsilver (Ag), titanium dioxide (TiO₂), and carbon (C) in one or moreembodiments.

In an embodiment, a method of forming a paper-based substrate memorydevice is provided. The method can comprise: coating one or more areasof a paper substrate with a conductor material to form a first electrodeof the memory device; depositing, with the least one printer, a layer ofan insulator material over one or more areas of the conductor material;and depositing, with the least one printer, a layer of a metal over oneor more areas of the insulator material to form a second electrode ofthe memory device.

In any one or more aspects, the coating of the one or more areas of thepaper substrate can comprise coating and curing a plurality ofindividual layers of the conductor material over the one or more areasof the paper substrate. The number of the plurality of individual layersof the conductor material can comprise, for example, between 8 and 12. Ascreen printer can be used for coating the conductor material upon theone or more areas of a paper substrate. The depositing of the layer ofinsulator material can comprise depositing a plurality of layers of theinsulator material over the one or more areas of the conductor material.An ink jet printer can be used for depositing the layer of insulatormaterial over the one or more areas of the conductor material. An inkjet printer can be used for depositing the layer of the metal over theone or more areas of the insulator material. The conductor material canbe selected from the group consisting of carbon, copper, aluminum,nickel, gold, silver, titanium, germanium, platinum and palladium. Theconductor material can be a carbon paste. The insulator material can bea semiconductor material. The insulator material can be selected fromthe group consisting of titanium dioxide, copper, aluminum, nickel,gold, silver, titanium, germanium, platinum and palladium. The insulatormaterial can comprise a titanium dioxide ink comprising TiO₂nanoparticles, acetyl acetone, Triton-X-100, distilled water, ethanol,and ethylene glycol. The metal can be a metal having mobile metal ions.The metal can be selected from the group consisting of silver, copper,nickel, zinc and gold. The metal can comprise a silver ink comprising Agnanoparticles, ethylene glycol, and water. The method can furthercomprise the step of sonicating the silver ink before printing thesilver ink with the ink jet printer.

In an embodiment, a paper-based substrate memory device is provided. Thepaper-based substrate memory can comprise: a paper substrate coated witha conductor material; a layer of insulator material deposited over oneor more areas of the conductor material, the layer of insulator materialhaving a thickness of more than 40 μm to less than 100 μm; and a layerof metal deposited over one or more areas of the insulator material. Inany one or more aspects, the conductor material can form a firstelectrode of the memory device; and the metal can form a secondelectrode of the memory device. The conductor material can be selectedfrom the group consisting of carbon, copper, aluminum, nickel, gold,silver, titanium, germanium, platinum and palladium. The conductormaterial can be a carbon paste. The insulator material can be asemiconductor material. The insulator material can be selected from thegroup consisting of titanium dioxide, copper, aluminum, nickel, gold,silver, titanium, germanium, platinum and palladium. The insulatormaterial can comprise a titanium dioxide ink comprising TiO₂nanoparticles, acetyl acetone, Triton-X-100, distilled water, ethanol,and ethylene glycol. The metal can be a metal having mobile metal ions.The metal can be selected from the group consisting of silver, copper,nickel, zinc and gold. The metal can comprise a silver ink comprising Agnanoparticles, ethylene glycol, and water. The conductor can be carbonpaste and the paper substrate can be coated with the carbon paste usinga screen printer; the insulator can be titanium dioxide and the layer oftitanium dioxide can be deposited using at least one ink jet printer;and the metal can be silver and the layer of silver can be depositedusing the at least one ink jet printer.

In an embodiment, a method of forming a paper-based substrate memorydevice, for example a WORM memory device is provided. The method cancomprise coating one or more areas of a paper substrate with a conductormaterial to form a first electrode of the memory device; depositing,with the least one printer, a layer of an insulator material over one ormore areas of the conductor material; depositing, with at least oneprinter, at least one diode onto the insulator material; and depositing,with the least one printer, a layer of a metal over the at least onediode of the insulator material to form a second electrode of the memorydevice.

In any one or more aspects, the coating of the one or more areas of thepaper substrate can comprise coating and curing a plurality ofindividual layers of the conductor material over the one or more areasof the paper substrate, such as between 8 and 12 layers. A screenprinter can be used for coating the conductor material upon the one ormore areas of a paper substrate. Depositing the layer of insulatormaterial can comprise depositing a plurality of layers of the insulatormaterial over the one or more areas of the conductor material. An inkjet printer can be used for depositing the layer of insulator materialover the one or more areas of the conductor material, depositing thelayer of the metal over the one or more areas of the insulator material,or both. The conductor material can be selected from the groupconsisting of carbon, copper, aluminum, nickel, gold, silver, titanium,germanium, platinum and palladium. The conductor material is a carbonpaste. The insulator material can be a semiconductor material. Theinsulator material can be selected from the group consisting of titaniumdioxide, copper, aluminum, nickel, gold, silver, titanium, germanium,platinum and palladium. The insulator material can comprise a titaniumdioxide ink comprising TiO₂ nanoparticles, acetyl acetone, Triton-X-100,distilled water, ethanol, and ethylene glycol. The metal can be a metalhaving mobile metal ions. The metal can be selected from the groupconsisting of silver, copper, nickel, zinc and gold. The metal cancomprise a silver ink comprising Ag nanoparticles, ethylene glycol, andwater. The at least one diode can be deposited onto one or more one ormore selected areas of the insulator material, the diode deposited bydepositing a first material ink having a first fermi level onto theinsulator material and next depositing a second material ink having asecond fermi level onto the first material ink, the second fermi levelbeing different than the first fermi level and selected to form aschottky diode. The conductor layer can be deposited as a plurality ofstrips onto the paper substrate, the metal layer can be deposited as aplurality of strips over the conductor layer at a non-zero angle inrelation to the strips of the conductor layer to form a plurality ofjuxtaposed intersections between the strips of the conductor layer andthe metal layer, the insulator material can be deposited onto theconductor layer between the conductor layer and the metal layer withinthe intersections and a diode can be deposited onto the insulatormaterial between the insulator material and the metal layer strips ateach intersection.

In an embodiment, a paper-based substrate memory device for example aWORM memory device is provided. The memory device can, comprise a papersubstrate coated with a conductor material; a layer of insulatormaterial deposited over one or more areas of the conductor material, thelayer of insulator material having a thickness of more than 40 μm toless than 100 μm; at least one diode deposited over an area of theinsulator material; and a layer of metal deposited over the at least onediode.

In any one or more aspects, the conductor material can form a firstelectrode of the memory device; and the metal material can form a secondelectrode of the memory device. The conductor material can be selectedfrom the group consisting of carbon, copper, aluminum, nickel, gold,silver, titanium, germanium, platinum and palladium. The conductormaterial can be a carbon paste. The insulator material can be asemiconductor material. The insulator material can be selected from thegroup consisting of titanium dioxide, copper, aluminum, nickel, gold,silver, titanium, germanium, platinum and palladium. The metal can be ametal having mobile metal ions. The metal can be selected from the groupconsisting of silver, copper, nickel, zinc and gold. The conductor canbe carbon paste and the paper substrate can be coated with the carbonpaste using a screen printer; the insulator can be titanium dioxide andthe layer of titanium dioxide can be deposited using at least one inkjet printer; and the metal can be silver and the layer of silver can bedeposited using the at least one ink jet printer. The at least one diodecan be comprised of two different material inks, the material inkshaving a difference in a fermi level of the material inks to provide aschottky diode.

Other systems, methods, features, and advantages of the presentdisclosure will be or become apparent to one with skill in the art uponexamination of the following drawings and detailed description. It isintended that all such additional systems, methods, features, andadvantages be included within this description, be within the scope ofthe present disclosure, and be protected by the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the embodiments described hereinand the advantages thereof, reference is now made to the followingdescription, in conjunction with the accompanying figures, thecomponents in the drawings not necessarily to scale with emphasisinstead being placed upon clearly illustrating the principles of thepresent disclosure, briefly described as follows:

FIG. 1 illustrates a fabrication process for an example paper-basedmemory device according to various embodiments described herein.

FIG. 2 illustrates representative overhead and cross-sectionalphotographs of an example paper-based memory device according to variousembodiments described herein.

FIGS. 3A-D illustrate certain electrical characteristics of the examplepaper-based memory device in FIG. 1.

FIG. 4, frames (a)-(h) illustrate certain characteristics of the examplepaper-based memory device in FIG. 1 and associated circuitry during abending test.

FIG. 5, frames (a)-(d) illustrate certain characteristics of dataremoval for the example paper-based memory device in FIG. 1 duringvarious security tests.

FIG. 6A illustrates zeta potential as a function of pH values intitanium dioxide ink.

FIG. 6B illustrates the size of titanium dioxide clusters as a functionof pH values in titanium dioxide ink.

FIG. 7 illustrates the viscosity of the silver ink as a function ofsilver content represented in weight percentage.

FIG. 8A illustrates a thermo-gravimetric analysis.

FIG. 8B illustrates a differential thermal analysis for 10 wt % silverink heated in the air at a rate of 10° C./min.

FIG. 9 illustrates a comparison of resistivity between bulk silver andinkjet-printed silver as a function of sintering temperature.

FIG. 10A illustrates the memory window of paper memory as a function oftitanium dioxide thickness.

FIGS. 10B-D illustrate cumulative probability distribution functions ofhigh and low resistance states as a function of titanium dioxidethickness, for thickness of 50 μm, 60 μm, and 80 μm, respectively.

FIG. 11A illustrates a cumulative probability distribution of resetcurrent.

FIG. 11B illustrates a cumulative probability distribution of resetvoltage.

FIG. 12 illustrates curvature as a function of trench spacing.

FIG. 13 illustrates diodes in a crossbar structure of a RRAM to preventsneaky path.

FIGS. 14A and 14B illustrate an embodiment a WORM resistive memory bythe present all-printing process. FIG. 14A illustrates an embodiment ofa fabrication process in a crossbar structure combined with diodes. FIG.14B illustrates multi-states of resistance to increase the informationstorage capacity of the WORM memory.

DETAILED DESCRIPTION

Described below are various embodiments of the present systems andmethods for paper-based memory devices. Although particular embodimentsare described, those embodiments are mere exemplary implementations ofthe system and method. One skilled in the art will recognize otherembodiments are possible. All such embodiments are intended to fallwithin the scope of this disclosure. While the disclosure will now bedescribed in reference to the above drawings, there is no intent tolimit it to the embodiment or embodiments disclosed herein. On thecontrary, the intent is to cover all alternatives, modifications andequivalents included within the spirit and scope of the disclosure.

According to the embodiments described herein, paper-based substratememory devices (“paper-based memory devices,” “paper-based memory,” or“paper memory”) and methods of making the devices are described. Thepaper-based memory can be embodied as an all-printed paper memoryincluding a metal-insulator-conductor structure. Themetal-insulator-conductor structure can be formed from structure ofsilver (Ag), titanium dioxide (TiO₂), and carbon (C) in one or moreembodiments.

Such a printed paper-based memory device (PPMD) can exhibit highreliability in terms of cycling endurance and data retention, with atunable ON/OFF memory window (up to three orders of magnitude) viatuning the thickness of the TiO₂ layer. The PPMD also shows excellentdevice performance even under relatively extreme bending conditions,indicating its mechanical robustness. As fabricated on an adhesivelabel, for example, the PPMD is also capable of being tightly andreliably affixed to surfaces of any articles, including electronicdevices and living subjects, with high-performance non-volatilefunctionality. This feature facilitates the use of memory devices as acomponent in a flexible, wearable, and biocompatible electronic system.Further, as compared to the relatively complex, time-consuming tasks ofsecurely deleting data from conventional memory devices, such asdegaussing or software-based erasing/overwriting, the permanent removalof data from a PPMD can be achieved simply by burning or shredding thePPMD. Finally, the printing processes described for preparingpaper-based memory devices herein may be relied upon to produce memoryat an estimated price of only 0.0003 cent/bit. Thus, the PPMDs describedherein may facilitate the development of various types of more complexpaper-based circuits.

FIG. 1 illustrates a fabrication process for an example paper-basedmemory device 100 according to various embodiments described herein. Atreference numeral 110, the process includes coating or printing aconductor 104 upon one or more areas of the paper substrate 102. Theconductor material can serve as a bottom electrode for the memorydevice. A suitable conductor is carbon that can be applied upon thepaper substrate as carbon paste. Other materials suitable as theconductor include copper, aluminum, nickel, gold, silver, titanium,germanium, platinum and palladium. The conductor 104 can form a type ofelectrode in the paper-based memory device 100.

In an embodiment, carbon paste 104 can be coated or printed upon thepaper substrate 102 by way of screen printing, although other methods ofprinting may be relied upon. Among embodiments, the paper substrate 102can be embodied as any suitable type of commercially available printingpaper. The type of paper used for the paper substrate 102 can beselected based on various considerations, such as grade, smoothness,foldability, cost, availability, or other factors.

The carbon paste 104 can be embodied as any suitable carbon pasteconsisting, at least in part, of a mixture of graphite and a pastingliquid. In this context, the carbon paste 104 can be empiricallyformulated (i.e., mixed) in the laboratory for certain electricalcharacteristics, for example, or a commercially available carbon pastecan be relied upon (e.g., carbon paste product of Acheson-Henkel, etc.).Any suitable screen printing machines, mechanisms, and processes can beused. For example, a disk or drum screen printing machine can be reliedupon, including but not limited to those manufactured by Houn Jien Co.,Ltd. of Taiwan (e.g., HJ-55ACR, HJ-55AJ1, HJ-55AC1, etc.). The screenmesh used for screen printing can vary in material and mesh count. Inone embodiment, the screen mesh can be formed from polyester having thefollowing properties: mesh count 150 mesh/inch; mesh opening 100 μm;thread diameter 50 μm; open surface 35%; fabric thickness 80 μm.

At reference numeral 110, the process can include applying severalindividual coats of the conductor 104 on the paper substrate 102,followed by curing the conductor 104 between coats in the case forexample of applying carbon paste. The curing can be performed for asuitable period of time to permit the carbon paste 104 to dry (e.g., 5,10, or 15 minutes, etc.). To speed the curing process, the papersubstrate 102 and carbon paste 104 can be cured in an oven or vacuumoven at a temperature of about 100° C. or another suitable temperature.The number of coats can range depending upon the roughness of the papersubstrate 102, for example, or other factors. In one preferredembodiment, ten coats of the carbon paste 104 may be applied on thepaper substrate 120, but it should be appreciated that greater or fewercoats may be applied.

At reference numeral 112, the process can include depositing aninsulator over one or more areas of the conductor 102. The insulator canserve, in cooperation with the metal layer (described below), as anactive layer, or the switching medium or layer. As a non-limitingexample the process can include depositing a layer of titanium dioxide106 as the insulator. The titanium dioxide can be deposited in an inkmixture (TiO₂ ink), over one or more areas of the carbon paste 104 byway of inkjet printing. The TiO₂ ink can be prepared, for example, bymixing 0.5-g TiO₂ nanoparticles of about 25 nm in diameter (e.g., bySigma-Aldrich Co. LLC.) into a solvent of 50-μl acetyl acetone (e.g., byAlfa Aesar), 50-μl Triton-X-100 (e.g., by ACROS), 8.5-ml distilledwater, 1-ml ethanol, and 0.5-ml ethylene glycol. A MicroFab JetLab4inkjet printer system (e.g., by MicroFab Technologies, Inc.) with twopiezoelectric nozzles of 50 μm in diameter, or other suitable inkjetprinter systems having suitable piezoelectric nozzles, can be used fordepositing the layer of titanium dioxide 106. The TiO₂ ink can bedeposited at a temperature of about 50° C., but other temperatures maybe relied upon. At reference numeral 112, the process can includeapplying several individual coats of the titanium dioxide 106 over thecarbon paste 104, with or without permitting time for drying or curingbetween coats.

Semiconductor materials can also be used for the insulator 106.Semiconductor materials suitable as the insulator or active layerinclude zinc oxide, copper oxide, nickel oxide, molybdenum disulfide,gallium nitride, etc.

After the one or more layers of the insulator 106 have been applied, theprocess can include depositing a layer of metal 108 over one or moreareas of the insulator. The metal layer can serve as a top electrode forthe memory device. As a non-limiting example, the process can includedepositing a layer of silver as the metal 108. The silver can bedeposited, in an ink mixture (Ag ink) over one or more areas of theinsulator 106 (e.g., titanium dioxide) by way of inkjet printing atreference numeral 114. The metal 108 can form a type of electrode in thepaper-based memory device 100. A MicroFab JetLab4 inkjet printer system(e.g., by MicroFab Technologies, Inc.) with two piezoelectric nozzles of50 μm in diameter, or other suitable inkjet printer systems havingsuitable piezoelectric nozzles, can be used for depositing the layer ofsilver 108. The diameter of the piezoelectric nozzles of the inkjetprinter system may be selected to achieve a size and velocity of ejecteddroplets of the TiO₂ and Ag inks of about 50 μm, at 1000 Hz, withejection velocities of 1.86 m/s and 2.6 m/s for the TiO₂ and Ag inks,respectively. In one or more aspects a suitable metal is one that canionize with the application of voltage.

Other metals that can be used for the metal layer 108 include metalsthat have mobile metal ions (MMI). Metals suitable for the metal layer108 that have mobile metal ions include copper, nickel, zinc and gold.

At reference numeral 114, the process can also include applying severalindividual coats of the metal 108, for example silver, with or withoutpermitting time for drying or curing between coats. In one embodiment,the silver 108 can be cured by a sintering process at 180° C. for 1hour, although the sintering process may be performed at othertemperatures and/or for other periods of time.

A suitable Ag ink can be prepared, for example, by adding about 10 wt %Ag nanoparticles into a humectant solution (e.g., 30% ethylene glycoland 70% water). The Ag nanoparticles can have a mean diameter of about200 nm, for example, or another suitable diameter. To help disperse theAg nanoparticles in the Ag ink, the ink can be put into a sonicationbath for 2 hours. Thus, at reference numeral 114, the process caninclude sonicating the Ag ink for a period of time.

The use of inkjet printing in the process illustrated in FIG. 1 providescertain advantages. For example, the patterns of memory cells can becontrolled with a high degree of freedom without relying uponlithography technique. In this context, FIG. 2 illustratesrepresentative overhead and cross-sectional photographs of an examplepaper-based memory device according to various embodiments describedherein. As shown in FIG. 2, memory cells can be printed as alphabeticalletters 202 or dot arrays 204. Further, using inkjet printing, eachlayer can be deposited on paper substrates with high uniformity. Thestructural uniformity of the embodiments was confirmed by scanningelectron microscopy, as shown in the cross-sectional photograph in FIG.2. The lack of agglomerations in the cross-sectional photographindicates good dispersion of each printed layer.

Because working with the relative roughness of paper substrates isnecessary when fabricating printed electronic components on paper,additional coatings or polishing processes were usually required toachieve a smoother and less non-absorptive substrate surface. Suchprocesses generally resulted in cost and recyclability compromises,however. One advantage to the process described in FIG. 1 is that,through the sequential printing processes, the roughness of papersubstrates can be gradually smoothed. Additionally, as with conventionalinkjet printing, each printed layer can be tightly embedded onto paper.Thus, the paper-based memory devices described herein can retain goodflexibility.

With inkjet printing with a dot size of about 50 μm and pitch resolutionof about 25 one memory cell can occupy a square of about 100 μm on eachside, or 10⁴ bit/cm². At that density, a fully printed piece of A4 papercan store about 1 Mb of data. Using a finer inkjet printing process(e.g., super-fine ink jet (SIJ) technology), it is possible to achieve adot resolution of <1 μm at nearly the same pitch width. Thus, thedensity can be enhanced by about 2500 times, and >1 Gb of data can beobtained on one A4 paper.

Turning to a discussion of the operating characteristics of apaper-based memory device prepared using the process in FIG. 1, FIGS.3A-D illustrate certain electrical characteristics of the paper-basedmemory device. FIG. 3A provides typical current-voltage (I-V) switchingcharacteristics of the paper-based memory device. After preparation,paper-based memory devices are in a high resistance (HRS or OFF) stateand can be directly operated without an electroforming process, whichmay be beneficial from the viewpoint of RRAM circuit operation. Withoutelectroforming (e.g., forming or changing the resistive state), thepaper-based memory devices described herein show relatively uniformperformance and reproducible switching operation by both DC voltagesweeping and AC voltage pulses.

As for electroforming, by applying a positive set voltage bias exceedingabout 1 V, the current across the paper-based memory device abruptlyincreases with a resistive switch from HRS to low resistance (LRS orOFF) state. In the paper-based memory device, the ON state is retainedafter the applied electroforming bias is turned off, exhibiting anon-volatile memory behavior. To turn the device OFF, a negativeelectroforming bias is applied (e.g., reset voltage of about −3V),inducing a decrease of current and turning the memory device back to theHRS. In one or more aspects, a voltage in the range of about −3V toabout +2V can be applied to store data. The switching mechanism followsfrom electrochemical metallization, due at least in part toelectrochemical dissolution of a mobile metal (e.g. Ag) to perform theON/OFF switching operation. The state of the TiO₂ layer cannot itself beswitched without the deposition of the inkjet-printed Ag electrodes,which demonstrates the role of Ag as the source for conductingnanofilaments.

The Ag/TiO₂/C paper-based memory device described herein can be referredto as a conductive-nanobridge RRAM, recognized as the formation and therupture of the Ag conductive nanobridge within the dielectric TiO₂electrolyte. As such, a porous structure featuring high permeability formetal atoms/ions can be preferable. Since the TiO₂ ink is made ofnanoparticles, the structure is naturally porous and thus provides morechances to form the conductive nanobridges. This can result in aforming-free characteristic of the paper-based memory devices with adevice yield of over about 90%.

An important consideration for PPMDs is their tunable memory window. Thememory window can be tuned by modulation of the thickness of the TiO₂layer, as controlled using inkjet printing times, for example. FIG. 3Bshows the high (HRS) and low (LRS) resistance state values as a functionof thickness of printing Tio₂ layers, with the ON/OFF ratio (the memorywindow) ranges from 10 to 10³. It shows the TiO₂ layer can be <100 μm.In other aspects, the TiO₂ layer can be in the range of 60 μm to 80 μm.

It shows that a thin TiO₂ layer (e.g., thickness<40 μm) results in nomemory window, with the device in the LRS state due to the Ag electrodespenetrating directly through the TiO₂ layer to the C electrodes. Whenthe TiO₂ layer is thick enough (e.g., thickness>50 μm), the memorywindow appears and enlarges with TiO₂ layer thickness. As the TiO₂ layerthickness is more than about 100 μm, the devices become pure insulatorsand cannot be operated. In one or more aspects the thickness of theinsulator layer can range from 40 μm to less than 100 μm. In one or moreother aspects the thickness of the insulator layer can range from lessthan 100 μm to more than 40 μm. In other aspects, the thickness of theinsulator layer can range from 45 μm to 95 μm, 50 μm to 90 μm, or 55 μmto 85 μm. Taking advantage of the tunable nature of inkjet printingtechniques, the tunable window provides a broad range of reading marginsaccording to various requirements and sensitivities of systems. A TiO₂layer of about 50 μm or more in thickness can be preferred to allowpaper-based memory devices to perform with a high ON/OFF memory window.

As to programmable capability and endurance, a PPMD was successivelyswitched between HRS and LRS states 100 times during an endurance Test,and the results are shown in shown in FIG. 3C. During the 100 endurancecycles, both the HRS and LRS states retained their resistance valueswithout a significant change in reading bias, showing reproducible andrepeatable switching capability. This state retention property was alsocharacterized at 85° C., and the results are shown in FIG. 3D. Theresistance ratio of resistive states were retained up to 3×10⁴ s at 85°C. without degradation, showing that the PPMD is capable of maintainingdata integrity under harsh environment of relatively high temperatures.In testing, PPMDs are capable of maintaining data integrity attemperatures up to 150° C., as described in further detail below.

The flexibility of PPMDs, for foldable and wearable applications, is ofsome practical importance. As shown in frame (a) of FIG. 4, a bendingtest was performed to verify if PPMDs can be reliably operated underbent conditions. The bending test was conducted by mounting the deviceson homemade stages with confined gaps. The degree of bending isexpressed by the radius of curvature (r) between two edges of thesubstrate, as shown in the inset of frame (a). Cycle-to-cycle anddevice-to-device resistive switching behaviors were tested for 5 cells.For r>20 mm, the same switching characteristics as that for a flatcondition were observed. For r=10 mm, the switch window decreased byabout 20% relative to the flat condition. It is noted, however, that thememory window is reversible as a PPMD returns from r=10 mm to the flatcondition. To highlight the mechanical robustness of PPMDs, memoryperformances were monitored after repetitive bending for more than 1000times with a bending radius of about ˜10 mm, and the results are shownin frame (b) of FIG. 4. The suitable stability shown in frame (b), evenafter successive bending with r=10 mm, confirms the reliability ofPPMDs.

The ability of PPMDs to be labeled or affixed on surfaces of variousarticles can be another important feature. In this context, PPMDs werefabricated on an adhesive label using the process described in FIG. 1,and then affixed to different objects. The PPMDs were tested andtriggered by AC pulses after adhesion. The equivalent circuit is shownin frame (c) of FIG. 4. The writing bias is a positive pulse of 6 V andwidth of 100 μs, and the erasing bias is a negative pulse of 3 V andwidth of 200 μs. The reading voltage is 0.5 V. Operating voltages wereapplied on the top silver electrode, and the bottom carbon electrode wasgrounded. During the measurement, the positive bias is defined ascurrent flow with direction from the top to the bottom electrodes, andthe negative bias was defined as the direction from the bottom to thetop electrodes. A resistance of 500Ω was applied to restrain the current(compliance current) flowing through the devices. The electrical pulsewas trigged by a function generator, and the resistance was measured bya power-meter.

Frames (d) and (e) of FIG. 4 show the switching endurance of PPMDslabels on different solid surfaces, including a smart phone (r=∞, flatsurface) and a battery (r=14 mm). As the input voltage pulses wereapplied for 1000 times, the resistive values responding to ON and OFFstates were recorded. The ON/OFF ratio for the flat surface in frame (d)is slightly larger than the curved surface in frame (e). The resultindicates that the PPMD can be readily implemented into planar ornon-planar electronic devices with stable switching properties.

As shown in frames (f)-(h) of FIG. 4, the use of PPMDs on human skin wastested. Because paper substrates are formed of cellulose, they arebiocompatible and suitable for wearable applications. The storagecapability of PPMDs affixed to the back of the hand was shown to besuccessful, as demonstrated in frame (f) of FIG. 4. PPMDs stuck toun-deformed skin also show stable switching behaviors for over 1000switching cycles, as demonstrated in frame (f). As shown in frame (h) ofFIG. 4, PPMDs on compressed and stretched skins are still switchable andoperate normally. Such tests demonstrate that PPMD stickers could enablethe use of memory devices as a component in a flexible, wearable, andbiocompatible electronic system.

To secure sensitive data, data removal can be an important issue,particularly in military or commercial uses. Typically, data eliminationis carried out by relatively complex and/or time-consuming physicaldestruction, insecure degaussing, and/or software-basederasing/overwriting. Owing to the mechanical robustness of semiconductormemories and advanced recovery techniques, permanent data eliminationmay be difficult to achieve in many conventional memory devices.

One advantage of the paper-based memory devices described herein isdisposability. Here, two examples are shown to demonstrate the ease ofdata removal for paper-based memory devices. First, the data can betotally and irreversibly eliminated by heating a PPMD to over 250° C. origniting it. The temperature-dependent switching performance of a PPMDis shown in frame (a) of FIG. 5. Below 150° C., PPMDs can retain theON/OFF memory window without significant degradation, demonstratingtheir stability under extreme weather conditions. It is noted that papersubstrates do not deform as much as low-cost plastics upon heating,which can be an important issue for applications in harsh environments.At temperatures higher than 150° C., the memory window gradually shrinksand the difference between the ON/OFF states becomes indistinguishableas the resistance of the HRS and LRS states was decreased and increasedwith temperature, respectively. Then, at higher temperatures (>250° C.),the cellulose paper fibers darkened and decomposed within five seconds,leading to a permanent failure of the device, as shown in frame (b) ofFIG. 5.

Paper shredding is another efficient way to securely remove data fromPPMDs, as shown in frame (c) of FIG. 5. With traditional paper,shredders are relied upon to destroy documents. However, in some cases,shredded paper can be reconstructed by piecing together a series ofshredded segments of paper, using the original print as a guide. On theother hand, when PPMDs are shredded, each piece or segment is likely tolook substantially the same, as shown in frame (c). Thus, it isdifficult to reconstruct the PPMD. Frame (d) in FIG. 5 is an exampleshowing the I-V characteristics of a PPMD before and after shredding. APPMD was tested to ensure its operation status before shredding. Then,the PPDM was shredded by a shredder and measured again. The paper memorydevice failed to switch and showed the nature of pure resistance (˜58Ω)after shredding, which is even lower than the LRS before shredding,indicating a leakage through the TiO₂ nanoparticle layer. This resultshows that the shredding can lead to permanent data elimination.

The use of paper-based electronics is motivated by the ability for rapidmanufacturing and cost effectiveness, among other factors. For instance,paper is produced at a speed exceeding 10⁶ m²/hour and at a cost ofabout $0.06 cent/inch², which is about 5 orders faster and 3-4 orderscheaper than those of monocrystalline Si wafers, the most commonly-usedsubstrates in the electronic industry. Combining the utilization ofpaper substrates with printing techniques without the need forlithography leads to a time- and cost-effective manufacturing scheme forpaper memory. Moreover, the paper-based substrate memory devicesdescribed herein have an estimated cost of about 0.0003 cent/bit. Ascompared to other devices, paper RFID tags (1 bit) fabricated onpolyethylene terephthalate (PET) substrates are estimated to cost about3 cent/bit, credit-card-sized ID tags (96 bits) on paper have a cost ofabout 0.021 cent/bit, and a sensors or authentication systems on papersubstrates cost about 0.1 to 1 cent. These higher costs may beattributed to complex fabrication processes and/or larger areas perdevice, for example. The time- and cost-effective scheme of all-printedpaper memory employed according to the processes described herein offersa good replacement to those devices.

As outlined above, a non-volatile memory on paper is described using anall-printing approach. The paper-based memory shows good rewritableswitching properties and the capability to retain information at varioustemperatures. The paper-based memory exhibits stable endurance underbending conditions, demonstrating the characteristics of flexibleelectronics. Memory labels were also fabricated and affixed to otherelectronic devices and biological objects, showing integrative andbiocompatible characteristics. Finally, secure data disposal or removalwas demonstrated using the paper-based memory.

In a further aspect of our paper-based memory devices, we can providewrite once read many (WORM) memory devices. WORM memory allows messagesto be written in memory a single time and prevents data erasing. Thememory are deliberately not rewritable. They are intended, for example,to store data for a purpose in which the data is not to be tampered orerased by accident. Because of this feature, WORM memory have long beenused for the archival of organizations such as government agencies orlarge enterprises. In addition, WORM memory can work as a driver on asystem setting in a computer or circuit.

In one or more aspects, extending our all-printed paper memory fromsingle cell to multibyte WORM paper-based memory (see, e.g., FIG. 14B)can be realized through the design of a crossbar structure combined withdiodes which can be accomplished by our all-printing process. The stateof resistance in the WORM memory can be decided by adjusting thethickness of the switching medium (i.e, the insulator) or changing theinsulator materials during the printing process. A diode WORM memorystructure can be associated with each memory cell to prevent a sneakypath (or sneak path) condition from occurring in the crossbar structure.It can also provide a certain and fixed signal to drive devices in casesof system setting. The design of WORM resistive memory by ourall-printing process can satisfy the demands in system setting and havethe benefit of mass storage, energy saving and direct driving.Furthermore, the paper-based memory fabricated by our all-printingapproach can allow mass production. For example, in setting up roll toroll equipment, the memory device(s) can be completely fabricated by asequence of printing processes, which are low-cost and rapid. Wedescribe below how our single-cell paper memory can be extrapolated toprovide WORM memory devices.

In our discussion above we demonstrated how nonvolatile RRAM electronicscan be combined with paper to provide a paper-based memory withmetal-insulator-conductor structures. The RRAM exhibits abipolar-switching behavior. The ON/OFF switching operation of the memorycan be controlled by the electrochemical metallization. The paper-basedmemory has many advantages, one being the tightly embedding of eachprinted layer on the paper substrate allowing great flexibility of thedevice.

FIG. 1 described in more detail above, shows a schematic of an aspect ofour paper-based memory. First, a conductor material (for example, carbonpaste) can be coated on a paper substrate. The conductor material canserve as the bottom electrode(s). Next, an insulator material orswitching medium (for example, TiO₂ nanoparticles) can be printed on theconductor material using an inkjet printing process. After the insulatorlayer, the metal layer (for example, silver nanoparticle inks) can bedeposited. The metal layer can serve as the top electrode. Infabricating printing electronic components on paper, the roughness issuecan be important. Generally, additional polishing or coating processesare often required to get a smooth and non-absorptive surface. However,it would increase the cost and loss recyclability in the same time. Inour case, the roughness is progressively smoothened as inkjet-printinglayers on top of the paper substrate by a sequential printing approach(FIG. 2), which is hard for any single printing processes. This conceptcan be applied to our WORM memory devices.

Generally speaking, scaling down the dimensions of a memory cell can beadvantageous for commercial applications. Effectively dividing thematerials to isolate neighbor cells in crossbar arrays can work well inadvanced cell architecture, for example in high cell density crossbararrays in plane structure. The characteristics of high cell density alsocorrespond to minimizing the feature size of the device. Nevertheless, acrossbar array may lead to unwanted current leakage paths which are alsocalled sneaky path. The sneaky path can result in the misreading ofmemory state(s) which can significantly influence the operation of thememory. In order to solve this problem, additional diodes can becombined with a memory cell. The resistance of sneaky path can bedramatically increased by the barriers from the diodes. As a result, thecircuit system can precisely read the resistance states of the memory.

An example of the sneaky path in a memory crossbar structure is shown bythe line 13 in FIG. 13. For the example in the memory architecture 10 ofFIG. 13, different wordlines 22 observe different small differences involtage (e.g., because the program pattern in the array is different).For instance, the middle wordline connected to selected RRAM memory cell10 a can experiences a given voltage, whereas the top and bottomwordlines 24 connected only to un-selected RRAM memory cells canexperience a lower voltage. This difference in voltage can result in oneor more several sneak path currents, such as path current 13, throughoutmemory architecture 10.

The crossbar memory array 10 can include a parallel array of bottomelectrodes 11 extending along a first direction. In an embodiment, thebottom electrodes 11 can include the bottom conductor material. Aparallel array of top electrodes 15 can extend along a second directionjuxtaposed above and intersecting the bottom electrodes 11. The topelectrodes 15 can include the metal layer capable of providing mobilemetal ions. In an embodiment, the top electrodes and the bottomelectrodes are orthogonal to each other.

Each intersection of the two electrode arrays can define a two-terminalresistive memory cell. The memory cell at each intersection can includethe two electrodes 11, 15 separated by a switching medium (insulatinglayer). The switching layer can include the insulating medium, and inthe WORM memory embodiment also a diode 17. The switching layer orstructure can be the same width or narrower than the bottom electrode11. In some embodiments, each memory cell in a crossbar memory array canstore a single bit; in other embodiments, the memory cells can exhibitmulti-level resistance thereby allowing storage of a plurality of bitsat each cell, as depicted in FIG. 14B.

To promote all printed paper memory into mass production, plural storagespaces can be advantageous for use in paper-based electronics. Thecrossbar structure of FIG. 13 can be a solution for providing largeamounts of database. To drive the paper device, it usually requires afixed signal in system setting. We provide a WORM resistive memory byour all-printing process. The resistance state of the WORM can bedetermined by the thickness of active layers during the printingprocess. The various resistance states of each cell in the crossbarstructure can be combined into a particular output signal, which candrive the paper device directly without writing memory. Implantingdiodes 17 in each memory cell can prevent the sneaky path from occurringin the crossbar structure. Preferably, the operating voltage of thememory cell and the diode should match each other. A lower barrier ofthe diode can lead to the misreading of memory states caused by a sneakypath, and a higher barrier can result in the failure to switchresistance state.

To achieve this objective, we can quantify the resistive states of theinsulator layer of the memory device (for example, TiO₂ or othersemiconductors layer) in the memory cell as a function of thickness,which can be controlled by printing times. According to the outputsignal, the resistance and thickness of active layer can be determinedand obtained. Then, a suitable diode can be fabricated as a selector toprevent the cross-talk during reading. I-V characteristics of the diodecan be adjusted and tested to make sure the operating voltages arematched between memory cell and the diode. After the explicitmeasurement, the WORM memory can be realized by ink-jet printing in, forexample, a crossbar structure. Moreover, WORM memory can furtherincrease the information storage capacity by establishing differentthicknesses of the insulator layer, which can result in the multi-statesof resistance in memory cell.

An embodiment of a fabrication process and corresponding multi-states ofWORM memory are shown in FIG. 14. A paper substrate can be provided asdescribed above. Conductor material can then be coated or deposited ontothe paper substrate, as shown in FIG. 14A.1. The conductor material canbe as described above and applied as described above. In one or moreaspects the conductor material, forming the bottom or first electrode(s)can be deposited onto the paper substrate in the form of a plurality ofstrips. These strips can, preferably, be deposited parallel to eachother. Insulator material, such as described above, can then bedeposited on one or more areas or locations of the conductor material orbottom electrodes, as depicted in FIG. 14A.2. The material and themanner of application or depositing of the insulator material can alsobe in the manner as described above. At least one diode, and preferablya plurality of diodes, can be printed onto the insulator material, asdepicted in FIG. 14A.3. The one or more diodes can be formed of at leasttwo material inks, the inks, the two inks having a different fermi levelbetween them so as to build or provide a schottky diode. Since thediodes can be comprised of inks having different fermi levels theprinting of the diodes can be in two steps in which a first ink having afirst fermi level is printed onto the insulator material and then asecond ink having a second fermi level is printed onto the first fermilevel ink, thereby forming the one or more diodes. A metal layer can bedeposited or printed over the one or more diodes, such as depicted inFIG. 14A.4. The metal layer provides a conductor material to form secondelectrode of the memory device. The metal layer can be comprised ofmaterials and deposited as described above. In one or more aspects, themetal layer, identified as top electrodes in FIG. 14A.4, is juxtaposedover the bottom conductor material (first electrodes) in a manner toform a plurality of juxtaposed intersections between the top and bottomelectrodes. In an aspect the top and bottom electrodes can each beformed as a plurality of strips juxtaposed at a non-zero angle inrelation to each other. In an aspect the top and bottom electrodes caneach be formed as a plurality of strips juxtaposed at a non-zero angleto each other. In an aspect the top and bottom electrodes can each beformed as a plurality of parallel strips juxtaposed at right angles toeach other. The insulator material and diode(s) can be provided betweenthe top and bottom electrodes at one or more of the juxtaposedintersections, such as depicted in FIG. 14A.4, thereby forming apaper-based all printed WORM memory device.

Turning to FIG. 14B, a WORM memory array including multiple states ofresistance among the cells in the array is shown. Using multiple statesof resistance among the cells can increase the capacity of informationstorage. To realize this concept, first, the resistive states of theTiO₂ (or other semiconductor material) layer in the array are quantifiedand/or characterized for resistance as a function of TiO₂ layerthickness. TiO₂ layer thickness can be controlled by printing times, forexample.

Referring back to FIG. 10A, representative memory windows of cells as afunction of TiO₂ layer thickness are shown. Specifically, as shown inFIG. 10A, the operating memory window (the ratio of HRS and LRS) ofcells with TiO₂ thicknesses of 50, 60, and 80 μm are 6.2, 857.3, and985.7, respectively. Thus, using different TiO₂ thicknesses forindividual cells in a WORM memory array results in different programmed“states” for cells having different TiO₂ thicknesses.

Thus, as one example, the WORM memory array shown in FIG. 14B can berealized by using a 55 μm-thick TiO₂ layer for the “00” cells, a 65μm-thick TiO₂ layer for the “01” cells, a 75 μm-thick TiO₂ layer for the“10” cells, and a 85 μm-thick TiO₂ layer for the “11” cells. Each of the55 μm, 65 μm, 75 μm, and 85 μm TiO₂ layers corresponds to a differentHRS resistance state and, accordingly, bit combination. Beforeelectroforming (e.g., forming or changing the resistive state), thepaper-based memory cells and devices described herein show relativelyuniform performance and are in a high resistance (HRS or OFF) state. Asfor electroforming, by applying a positive set voltage bias exceedingabout 1 V, the current across the paper-based memory cell abruptlyincreases with a resistive switch from HRS to low resistance (LRS orOFF) state.

Additional Supporting Disclosure and Details

Additional details regarding the paper-based memory devices describedherein (and variations thereon) are provided below for furtherreference.

As for the zeta potential of titanium dioxide ink as a function of pHvalues, TiO₂ nanoparticle powder (e.g., Sigma System, Inc.), having adiameter of about 25 nm, was ground and mixed with distilled water. Themixture was dispersed in a sonication bath for several hours. Then, HCland NaOH solutions were added to adjust the pH value of the TiO₂ ink,and the zeta potential was characterized using a Zetasizer Nano system(Malvern Instrument) at a concentration of 0.025 wt %. The sedimentationrate of the nanoparticles was tested by a centrifuge at 3000 rpm.

FIG. 6A illustrates zeta potential as a function of pH values intitanium dioxide ink. The zeta potential is 40 mV at pH=2, and staysconstant until the pH value increases to 4. As the pH value increases,the value undergoes a transition from 40 mV (pH=4) to −40 mV (pH=6). Thezero charge point is found at pH=5.2, corresponding to the isoelectricpoint of the ink. A similar trend was also observed in the size ofcluster as a function of pH value. FIG. 6B illustrates the size oftitanium dioxide clusters as a function of pH values in titanium dioxideink. In FIG. 6B, the cluster size, or hydraulic diameter, is measured tobe about 200 nm for pH<4 and pH>6. Between pH=about 4 to 6, theaggregation of nanoparticle is more severe, leading to a relativelylarge cluster size (e.g., 500 nm). This may be attributed to a reductionof surface potential in this pH environment, where the electrostaticforces are less effective. Therefore, the TiO₂ can be adjusted to pH=4in some embodiments.

As for the viscosity of silver ink, Ethylene glycol can be used as asolvent to reduce the evaporation rate and avoid blocking at the nozzleof an ink jet printer. It can also modulate the viscosity of ink forprinting purposes. FIG. 7 illustrates the viscosity of the silver ink asa function of silver content represented in weight percentage. FIG. 7shows the viscosity of the silver ink at 25° C. as a function of silvercontent, and the result clearly indicates that the viscosity increaseswith silver content. In certain embodiments, 10 wt % of silver ink canbe employed with a viscosity of about 8 cp.

As for the sintering parameters of the silver ink, a thermo-gravimetricanalysis (TGA) and differential thermal analysis (DTA) were performed todetermine a suitable sintering temperature. FIG. 8A illustrates athermo-gravimetric analysis. Particularly, FIG. 8A shows the TGA for the10 wt % silver ink heated in the air at a rate of 10° C./min. Continuousweight loss was observed at the temperatures ranging from 25 to 300° C.FIG. 8B illustrates a differential thermal analysis for 10 wt % silverink heated in the air at a rate of 10° C./min. From the result shown inFIG. 8B, two broad valleys are presented in the curve, indicating thatthe solvents keep evaporating below 100° C. and 160° C. The two featuredtemperatures correspond to the boiling point of distilled water (100°C.) and ethylene glycol (160° C.). Therefore, in certain embodiments,the sintering temperature may be set at about 180° C.

As for the electrical characteristics of the inkjet-printed silver ink,FIG. 9 illustrates a comparison of resistivity between bulk silver andinkjet-printed silver as a function of sintering temperature. Theresistivity of the printed silver line (150 μm in length, 10 μm inwidth, and 300 μm in thickness) was characterized by a two-probemeasurement. The results show that the as-printed silver line isnonconductive after drying at about 50° C. After baking at a temperaturehigher than about 150° C. for 1 hour, the resistivity of the sampledrops to about 14 μΩcm, which is about 8.7-fold of the bulk silver (atabout 1.6 μΩcm). Thus, the resistivity of the inkjet-printed silverdecreases with the sintering temperature. In this context, to insure theconductivity of the silver electrode as well as the integrity of thepaper substrate, the sintering temperature for the fabrication of thememory devices in various embodiments may be about 180° C.

As to the switching parameters of the paper-based memory devicesdescribed herein, FIG. 10A illustrates the representative memory windowsof the devices as a function of titanium dioxide thickness. As shown inFIG. 10A, the operating memory window (the ratio of HRS and LRS) of thedevices with TiO₂ thicknesses of 50, 60, and 80 μm are 6.2, 857.3, and985.7, respectively. To verify the operating stability of the devices,cumulative probability distribution functions of the HRS and LRS statesfor the 50, 60, and 80 μm TiO₂ thickness devices were plotted, as shownin FIGS. 10B-D. All the devices show a concentrative distribution of LRSwith a small coefficient of variation (COV, standard deviation dividedby mean), while the cumulative probability of HRS for all devicesexhibits a clear distinction, as shown among FIGS. 10A-D.

The switching parameters of paper-based substrate memory devices arealso summarized in Table S1 below.

TABLE S1 TiO₂ thickness HRS/ (μm) HRS (Ω) LRS (Ω) LRS V_(reset) (V)I_(reset) (mA) 50 μ = 18.7k μ = 3.0k 6.2 μ = −3.7 μ = 22.3 COV = 24% COV= 44% 60 μ = 2.3M μ = 2.7k 851.9 μ = −3.2 μ = 18.4 COV = 173% COV = 36%80 μ = 6.1M μ = 6.2k 983.9 μ = −2.5 μ = 7.3 COV = 75% COV = 67%

The small COV for the LRS of all devices is evidence of the formation ofone or more conducting bridges capable to maintain relatively stableresistive states. For the HRS, the COVs for 50, 60, and 80 μm TiO2thickness devices are 24%, 173%, and 75%, respectively. The fluctuationof the HRS could be attributed to the roughness of the TiO₂ layer, andwith further optimization the fluctuation may be reduced. A TiO₂ layerwith a thickness of 80 μm that exhibits an acceptable COV may be used insome embodiments.

FIG. 11A illustrates a cumulative probability distribution of resetcurrent. More particularly, FIG. 11B shows reset current and voltage asa function of TiO₂ thickness. The distribution reveals that a thinnerTiO₂ layer requires higher reset voltage and current. Thus, it is notedthat an increase in thickness may lower the required voltage and currentand reduce the energy consumption needed for memory switching.

To perform the bending tests described herein, paper-based memorydevices were mounted on holders and characterized at the same time. Thepaper substrates were first trimmed to a length of about 30 mm and theninserted into trenches of the holders. To the extent that the spacing ofthe trenches was smaller than the length of paper substrates, the papersubstrates were curved. By varying the spacing of the trenches (l), thecurvature (r) of the flexible paper memory can be determined. Accordingto the relations: r=15 mm/sin⁻¹(½), the curvature as a function oftrench spacing is defined, as shown in FIG. 12.

As to the estimates of fabrication speed, modern paper machines canproduce a sheet of about 10 m in width and operate at speeds of morethan 100 km/h. This fabrication speed, in area, is 106 m²/h. Simonocrystals are fabricated with a crystal growth rate of about 1mm/min. For an 18-inch Si fabrication process, the expected thickness ofthe wafer is about 925 μm, which results in an estimated speed of 10m2/h.l

As to the estimates of cost, 1 g ($0.9) of silver nanoparticles wasmixed into 10 ml solvent to produce the silver ink. Each silver dropletis 50 μm in diameter, equivalent to a volume of 6×10⁻¹¹ L. Silverdroplets for two times are required for the formation of a topelectrode. The cost of the silver electrode is about 0.001 cent for abit. A 0.5 g ($0.06) weight of TiO₂ nanoparticles was mixed into 10 mlsolvent to produce the TiO₂ ink. Each TiO₂ droplet is 50 μm in diameter,equivalent to a volume of 6×10⁻¹¹ L. To fabricate the active layer, atleast about 50 TiO₂ droplets may be required for a bit, whichcorresponds to a cost of about 0.002 cent. Thus, the cost for a bit isabout 0.003 cent.

Ratios, concentrations, amounts, and other numerical data may beexpressed in a range format. It is to be understood that such a rangeformat is used for convenience and brevity, and should be interpreted ina flexible manner to include not only the numerical values explicitlyrecited as the limits of the range, but also to include all theindividual numerical values or sub-ranges encompassed within that rangeas if each numerical value and sub-range is explicitly recited. Toillustrate, a concentration range of “about 0.1% to about 5%” should beinterpreted to include not only the explicitly recited concentration ofabout 0.1% to about 5%, but also include individual concentrations(e.g., 1%, 2%, 3%, and 4%) and the sub-ranges (e.g., 0.5%, 1.1%, 2.2%,3.3%, and 4.4%) within the indicated range. In an embodiment, the term“about” can include traditional rounding according to significant figureof the numerical value. In addition, the phrase “about ‘x’ to ‘y’”includes “about ‘x’ to about ‘y’”.

Unless defined otherwise, all technical and scientific terms used havethe same meaning as commonly understood by one of ordinary skill in theart to which this disclosure belongs. Although any methods and materialssimilar or equivalent to those described can also be used in thepractice or testing of the present disclosure, the preferred methods andmaterials are now described.

All publications and patents cited in this specification areincorporated by reference as if each individual publication or patentwere specifically and individually indicated to be incorporated byreference and are incorporated by reference to disclose and describe themethods and/or materials in connection with which the publications arecited. The citation of any publication is for its disclosure prior tothe filing date and should not be construed as an admission that thepresent disclosure is not entitled to antedate such publication by priordisclosure. Further, the dates of publication provided could differ fromthe actual publication dates that may need to be independentlyconfirmed.

It should be emphasized that the above-described embodiments are merelyexamples of possible implementations. Many variations and modificationsmay be made to the above-described embodiments without departing fromthe principles of the present disclosure. All such modifications andvariations are intended to be included herein within the scope of thisdisclosure and protected by the following claims.

At least the following is claimed:
 1. A method of forming a paper-basedsubstrate memory device, comprising: coating one or more areas of apaper substrate with a conductor material to form a first electrode ofthe memory device; depositing, with the least one printer, a layer ofmaterial to serve as a switching medium over one or more areas of theconductor material; depositing, with at least one printer, at least onediode onto the switching medium layer; and depositing, with the leastone printer, a layer of a metal onto the at least one diode to form asecond electrode of the memory device, wherein the at least one diode isdeposited onto one or more selected areas of the switching medium layer,the diode deposited by depositing a first material ink having a firstfermi level onto the switching medium layer and next depositing a secondmaterial ink having a second fermi level onto the first material ink,the second fermi level being different than the first fermi level. 2.The method of claim 1, wherein coating the one or more areas of thepaper substrate comprises coating and curing a plurality of individuallayers of the conductor material over the one or more areas of the papersubstrate.
 3. The method of claim 1, further comprising using a screenprinter for coating the conductor material upon the one or more areas ofa paper substrate.
 4. The method of claim 1, wherein depositing theswitching medium layer comprises depositing a plurality of layers of theswitching medium layer over the one or more areas of the conductormaterial.
 5. The method of claim 1, further comprising using an ink jetprinter for depositing the switching medium layer over the one or moreareas of the conductor material, depositing the layer of the metal overthe one or more areas of the insulator material, or both.
 6. The methodof claim 1, wherein the conductor material is selected from the groupconsisting of carbon, copper, aluminum, nickel, gold, silver, titanium,germanium, platinum and palladium.
 7. The method of claim 1, wherein theconductor material is a carbon paste.
 8. The method of claim 1, whereinthe switching medium layer is a semiconductor material.
 9. The method ofclaim 1, wherein the switching medium layer is selected from the groupconsisting of titanium dioxide, zinc oxide, copper oxide, nickel oxide,molybdenum disulfide, and gallium nitride.
 10. The method of claim 1,wherein the switching medium layer comprises a titanium dioxide inkcomprising TiO₂ nanoparticles, acetyl acetone, Triton-X-100, distilledwater, ethanol, and ethylene glycol.
 11. The method of claim 1, whereinthe metal is a metal having mobile metal ions.
 12. The method of claim1, wherein the metal is selected from the group consisting of silver,copper, nickel, zinc and gold.
 13. The method of claim 1, wherein themetal comprises a silver ink comprising Ag nanoparticles, ethyleneglycol, and water.
 14. The method of claim 1, wherein the first fermilevel of the first material ink and the second fermi level of the secondmaterial ink are selected to form a Schottky diode.
 15. The method ofclaim 1, wherein the at least one printer is a screen printer, an inkjetprinter, a micro-contact printer, or a 3D printer.
 16. A method offorming a paper-based substrate memory device, comprising: coating oneor more areas of a paper substrate with a conductor material to form afirst electrode of the memory device; depositing, with the least oneprinter, a layer of material to serve as a switching medium over one ormore areas of the conductor material; depositing, with at least oneprinter, at least one diode onto the switching medium layer; anddepositing, with the least one printer, a layer of a metal over the atleast one diode to form a second electrode of the memory device; whereinthe conductor material is deposited as a plurality of strips onto thepaper substrate, the metal layer is deposited as a plurality of stripsover the conductor material at a non-zero angle in relation to thestrips of the conductor material to form a plurality of juxtaposedintersections between the strips of the conductor material and the metallayer, the switching medium layer is deposited onto the conductormaterial between the conductor material and the metal layer within theintersections and a diode is deposited onto the switching medium layerbetween the switching medium layer and the metal layer strips at eachintersection.
 17. The method of claim 16, wherein the thickness of theswitching medium at one juxtaposed intersection is thicker than thethickness of the switching medium at a second juxtaposed intersectionsuch that the one juxtaposed intersection and the second juxtaposedintersection have a different HRS resistance state.
 18. A paper-basedsubstrate memory device, comprising: a paper substrate coated with aconductor material; a layer of material to serve as a switching mediumdeposited over one or more areas of the conductor material, theswitching medium layer having a thickness of more than 40 μm to lessthan 100 μm; at least one diode deposited over an area of the switchingmedium layer; and a layer of metal deposited onto the at least onediode, wherein the at least one diode is comprised of two differentmaterial inks, the material inks having a difference in a fermi level ofthe material inks.
 19. The device of claim 18, wherein: the conductormaterial forms a first electrode of the memory device; and the metalforms a second electrode of the memory device.
 20. The device of claim18, wherein the conductor material is selected from the group consistingof carbon, copper, aluminum, nickel, gold, silver, titanium, germanium,platinum and palladium.
 21. The device of claim 18, wherein theconductor material is a carbon paste.
 22. The device of claim 18,wherein the switching medium layer is a semiconductor material.
 23. Thedevice of claim 18, wherein the switching medium layer is selected fromthe group consisting of titanium dioxide, zinc oxide, copper oxide,nickel oxide, molybdenum disulfide, and gallium nitride.
 24. The deviceof claim 18, wherein the metal is a metal having mobile metal ions. 25.The device of claim 18, wherein the metal is selected from the groupconsisting of silver, copper, nickel, zinc and gold.
 26. The device ofclaim 18, wherein: the conductor material is carbon paste and the papersubstrate is coated with the carbon paste using a screen printer; theswitching medium layer is titanium dioxide and the layer of titaniumdioxide is deposited using at least one ink jet printer; and the metallayer is silver and the layer of silver is deposited using the at leastone ink jet printer.
 27. The device of claim 18, wherein the materialinks have a difference in a fermi level of the material inks to providea Schottky diode.
 28. The device of claim 18, wherein the device is aWORM memory device.
 29. A method of forming a paper-based substratememory device, comprising: coating one or more areas of a papersubstrate with a conductor material to form a first electrode of thememory device; depositing, with the least one printer, a layer ofmaterial to serve as a switching medium over one or more areas of theconductor material; depositing, with at least one printer, at least onediode onto the switching medium layer; and depositing, with the leastone printer, a layer of a metal over the at least one diode to form asecond electrode of the memory device; wherein the conductor material isdeposited as a plurality of strips onto the paper substrate, the metallayer is deposited as a plurality of strips over the conductor materialat a non-zero angle in relation to the strips of the conductor materialto form a plurality of juxtaposed intersections between the strips ofthe conductor material and the metal layer, the switching medium layeris deposited onto the conductor material between the conductor materialand the metal layer within at least one intersection and a diode isdeposited onto the switching medium layer between the switching mediumlayer and the metal layer strips at the at least one intersection.